[gmx-users] Error in deuterium order parameters, index, nr

[Poncho Arvayo Zatarain]

Hello gromacs users: i´m trying to graph my xvg of deuterium order parameters with the command: gmx order -s file.tpr -f fiile.xtc -n sn1.ndx -d z -od deuter_sn1.xvg but the following error appears: Error in user input: Invalid input lines in option nr. Required option was not provided and the default file "index" does not exist or is not accessible. The following extensions were tried to complete the file name: .ndx.
Before that, to generate .ndx file i use the command: gmx make_ndx -f file.tpr -o sn1.ndx. What can i do to solve this? What is option nr? is necessary to use it?

Thanks